Ensuring that reacting species achieve optimal physical contact can be among the most difficult challenges in chemical reactor design. If done improperly, numerous undesired byproducts and an abundance of unreacted reactants can seriously erode the economics of the system. The volume of the reaction zone, the reactor type (i.e., batch, plug flow, stirred tank, or combinations thereof), thermal effects, reaction mechanism, reactant and product diffusion, pressure effects, and other factors must all be considered in selecting or fabricating a reactor best suited for use in a given reaction.
Of course, the nature of the reaction that is to occur in the reactor has much to do with the selection of the reactor. If a reaction mechanism only involves the bimolecular collision of small molecules, all that is desired is a contact between the two species at an energy state that provides a good chance for bonding to occur. Providing reactants with a particular residence time in a reactor may be necessary to increase the percentage of atomic or molecular collisions. However, if one or more of the reactants is capable of bonding at numerous sites greater residence time can also result in the production of numerous byproducts. This can be the case, for example, where diolefins are used as reactants. Thus, there is a balance that is sought between attaining complete reaction and overreacting or incorrectly reacting the reactants. The kinetics of reactions that involve more than simple bimolecular collisions are more complex and add to the factors one must consider.
Backmixing is another phenomenon which can contribute to further reaction of the reactor products. Backmixing is the mixing of a molecule or intermediate which has been present in the reactor for a given length of time with a molecule or intermediate which has been present in the reactor for a lesser period of time. The amount of backmixing that occurs is related to reactor geometry and type, fluid dynamics and other factors involved in reactor operations as noted above.
In commercial operations, the economic impact of a particular design is critical. These factors include the theoretical yield, side reactions, and process flow. Any process in competition with the desired reaction will result in a loss of value or an increase in costs due to recirculation of unreacted species and separation and treatment of byproduct. Other costs such as the cost of increased maintenance of equipment due to problems such as coke fouling can also appear.
It is well known that designing commercial chemical reactors is not amenable to a purely theoretical treatment. Typically, one begins the process by considering the reaction type (eg, reaction kinetics), catalytic requirements, phases involved, temperature and pressure effects on the reaction, production requirements, heat and mass transfer effects on the reaction, and secondary factors such as whether corrosion of the reaction vessel is likely. One then typically selects an ideal reactor that appears most applicable given these factors. For example, where the reaction mechanism suggests that back mixing would be particularly harmful, one may start with an analysis of an ideal plug flow reactor. Where back mixing is desired, a stirred tank may be selected.
Once an ideal reactor is selected, one then typically determines correction factors to account for deviations between the ideal and real behavior of the reaction. This is necessarily an experimental process. When the correction factors are determined, the reactor designer can then determine parameters such as reactor size and shape, whether the design (type) should be hybridized, and controls for parameters such as temperature and pressure. At this point, one may have the information that appears necessary to design an experimental reactor. Experimental reactors are then fabricated and tested.
The jump from the design of an experimental reactor to the design and production of a commercial or scaled-up reactor is necessarily a difficult one. For example, changes in reactor volume alone can greatly change operational parameters of what was previously thought to be a well understood system. Fluid dynamics, the nature of the reaction sites, reaction rates, and mass and heat transport considerations further complicate the problem.
The extent to which reaction conditions can be controlled is dictated, in large part, by the type of apparatus employed to conduct the reaction. Numerous reactors have been designed to solve particular problems. For example, U.S. Pat. No. 2,763,699 describes an apparatus for creating homogeneous turbulence of vapor phase reactants in curved reactors through the tangential positioning of injection nozzles about the inner surface of the vessel. It was found that use of the reactor therein described decreased the formation of carbon deposits that accompanies the production of allyl chloride from propylene and chlorine. Tangential injection essentially resulted in a two dimensional flow of reactants that traced the inner surface of the reactor.
U.S. Pat. No. 4,590,044 is an example of a reactor designed to ameliorate the effect of temperature variability in endothermic and exothermic reactions. This is done through the use of a number of baffles or reaction zones within the reactor.
Japanese Patent J73032087-B describes a reactor constructed specifically for the gas phase chlorination of hydrocarbons. The vessel used to mix the reactants is oblong having two opposing parallel flat surfaces and two opposing curved surfaces (when viewed in cross section). Jets are used to introduce the preheated reactant into the vessel tangentially and from opposing sides so that a swirl develops. This is said to allow increased heats of reaction and better mixing over previous reactor design. The reaction zone, or area in which the reaction occurred within the reactor, is essentially the entire inside volume of the reactor. While the tangential introduction of reactants creates a swirling effect, this effect is also predominantly a two dimensional effect. That is, swirling occurs essentially in one plane and traces the inner surface of the reactor. Further, since the reaction zone comprises essentially the entire inner volume of the reactor, there can be only one reaction zone.
Belgian Patent 742,356 describes a process for synthesizing allyl chloride by gas phase substitutive chlorination of propylene. The reactors used in this process incorporate a number of reaction zones. However, the process was designed specifically to avoid what the inventor viewed as the complex engineering necessary in reactors that incorporate a swirling or cycloning effect on reactants. Thus, here too, the reaction zone in such a system essentially comprises the entire inside volume of the reactor. The examples cited in the patent all employ a series of tubular reactors to accommodate this methodology.
Dykyj et al. describe a cyclonic reactor in High Temperature Chlorination of Propylene in a Cyclonic Reactor, International Chemical Engineering (Czecholoslovakia January, 1962). This design incorporates injection jets which introduce reactants tangentially to the inner surface of a cylindrical vessel. The jets are placed in opposition relative to the central axis of the reactor. However, they are offset so that they are not in direct opposition to each other. This causes the reactants to travel concentric to the inside surface of the reaction vessel so that a cyclonic effect is achieved. The authors assumed that no mixing would occur in the center part of the reactor and filled that portion of the reactor with a metallic core. Thus, the reaction zone in this arrangement appears as a cylinder with a hollow core (i.e. doughnut shaped). The movement of the reactants occurs in essentially two dimensions found in a circular plane with a hole in its center.
When a reactant is directed against a surface of the reactor, such as in the case of tangential injection, additional considerations arise. For example, if the reactant is a corrosive material such as chlorine, the reactor will frequently require special construction. One such method is to provide the interior surface of the reactor with a nickel coating. Such measures can dramatically increase the cost of the reactor.
Turbulent, swirling, and cyclonic flow reactors increase the likelihood of a collision between reacting species beyond what would be found if reactants were merely injected into a reactor without inducing motion on the reactants. However, prior art reactors generally induce motion in a single plane. At best, such reactors exhibit cocurrent mixing. This is predominantly a macromixing effect which folds layers or provides an overall flow to the stream of reactants. Some molecular collisions occur between the planes that are flowing but they are relatively few in number and occur by virtue of happenstance rather than design. The addition of heat and longer residence times can be used to induce such collisions but yield and selectivity losses generally accompany such measures as outlined above. In many instances adiabatic reaction design is preferred so adding heat to achieve greater frequency of collisions is not possible.
These problems are particularly acute where a number of possible reaction mechanisms may occur between reactants. The reaction between unsaturated hydrocarbons and halogens provides a good example of such a case. Either substitution reactions, addition reactions, or both may occur. Substitution reactions are preferred in the production of allyl chloride from propylene and chlorine. Higher temperatures are often necessary to create conditions more favorable to this substitution than addition reactions. Unfortunately, temperatures that are too high can result in the undue formation of coke and other undesirable substances and effects. Improperly increasing residence time might also create undesirable byproducts and reduce reaction selectivity.
Because the substitution reaction is preferred in the commercial preparation of allyl chloride as noted above, prior art reactors use increased temperatures to avoid the production of the by-product 1,2 dichloropropane (DCPo). These high temperature reactions are typically accompanied by the production of coke. The reaction may actually exhibit varying kinetic characteristics within one reactor because of the formation of hotspots. Numerous other reactions involving, for example, the selective chloro-substitution of ethylene, butylene, pentenes, hexenes, octenes, cyclohexene, acetylene, etc also experience such problems.
The art of reactor design could greatly benefit from the introduction of a reactor which would improve molecular contact, allow greater selectivity, and decrease reaction time/residence time substantially without the formation of coke and byproducts. This is particularly true in the case of the commercial preparation of allyl chloride from propylene and chlorine.